The invention relates to a plating method for filling a through hole of a printed board with a metal precipitated from a plating solution by electroless plating or electroplating.
Some of the multilayer printed circuit boards conventionally available have a built-up structure where a plurality of printed boards are multilayered on front and back surfaces of a core substrate. Describing steps of producing such a multilayer printed circuit board, a through hole is formed in the core substrate, an inner surface of the through hole is subjected to electroless plating or electroplating and then supplied with an electrically conductive paste, printed boards are multilayered on front and back surfaces of the core substrate, and required processes are repeatedly performed thereto. As a result of these steps, the multilayer printed circuit board is finally obtained.
In the multilayer printed circuit board thus produced, the printed boards are multilayered on the front and back surfaces of the core substrate, and the electrically conductive paste filling the through hole contains non-metallic components that lower a thermal conductivity. These factors deteriorate the heat dissipation capacity of the multilayer printed circuit board when, for example, heat is generated by electric current running through the through hole.
Conventionally, the heat dissipation capacity is often improved by filling the through hole of the core substrate with a metal by plating instead of using the electrically conductive paste as a filling material. For any through holes diametrically too small to be filled with the electrically conductive pastes, tilling such a through hole with a metal by plating is an effective solution that increases a degree of integration in the multilayer printed circuit boards.
However, filling the through hole of the core substrate with a metal by plating has the unsolved problem that voids and/or seams are easily generated in the metal used to fill the through hole. The void represents a phenomenon where air bubbles are trapped in a precipitated metal when parts of the precipitated metal separately growing from the side surface toward the axis of the through hole merge into each other near the axis. The seam represents a phenomenon where the precipitated metal merging near the axis of the through hole has some parts incompletely merged (often look like seams). These voids and seams are likely to cause breakage of wires when hit by thermal shock, possibly degrading the electric characteristics and the heat dissipation capacity of the through hole.
According to some techniques disclosed so far, the plating solution is modified as an attempt to prevent the voids and/or seams (for example, see the Patent Documents JP Patent Publication No. 2006-57177 and JP Patent Application Publication No. 2005-146343). These techniques, however, need to be refined in order to strictly control the voids and/or seams in any circumstances where the aspect ratio of the through hole is further increased.